Flow cytometric analyses of continuous BrdU incorporation patterns are being performed on radiation-sensitive and radiation-resistant lines of mouse cells at periods up to 24 h following doses of gamma radiation. These studies are designed to determine the extent of the sensitivity of the mouse cell lines following the transfection of new genes into the parental cell line. We have recently demonstrated that the transfection of specific genes can increase the radiation sensitivity of some cell line at different phases in the cell cycle.